1. Field of the Invention
The present invention relates to sewage pumping systems and, more particularly, to self-priming sewage pumping systems utilizing a sewage pump mounted above a reservoir storing sewage to be pumped.
2. Discussion of the Prior Art
It is well known in the art to use self-priming pumping systems for the movement of fluid sewage in that since such sewage pumping systems are primarily underground, it is incumbent that the sewage pump provides efficient operation for long periods of time without the need for frequent maintenance. In this regard, the requirement of low maintenance has required special adaptation of pumping systems for use in underground sewage treatment systems. The particular problems encountered in systems of this type include the complications resulting from the pumping of fluid material that contains certain amounts of sewage which can obstruct fluid flow passages, valves and other mechanical parts of the sewage pumping system. Further difficulties arise during the pumping of sewage due to its composition of liquid, semi-solid and solid matter in that decomposition and agitation of the sewage often causes the release of gases which accumulate in the pumping system.
U.S. Pat. No. 2,881,708 is exemplary of one type of prior art pumping system wherein a centrifugal pump does not run continuously, but operates intermittently, thereby requiring self-priming. This system utilizes a pumping apparatus including a combination of a vacuum pump and a centrifugal pump. The vacuum pump portion of the apparatus serves to bring water through an inlet tube into the centrifugal pump chamber and then into the delivery tube. The vacuum pump and centrifugal pump are driven by a single pump motor which turns a drive shaft connecting both the vacuum and centrifugal pumps. Therefore, in operation, when the pump is being primed, the centrifugal pump is in motion even though the pump chamber may be dry. Similarly, the vacuum pump is in constant operation when the centrifugal impellers are performing the fluid pumping operation. As a result of this design, the pumping apparatus, while providing self-priming, is subjected to undue wear and unnecessary simultaneous operation of the vacuum pump and centrifugal pump. Such wear is especially critical in the centrifugal portion of the pumping apparatus in that the centrifugal pump includes a mechanical seal to prevent leakage, which seal is designed to be lubricated by the fluid material when the system is in operation. Thus, during the priming operation, the dry condition of the seal can quickly destroy the seal and cause frequent maintenance to the pumping system. At the same time, the continuous operation of the vacuum pump while removing any air which accumulates in the system, causes an unnecessary drain on the pump motor and diminishes the efficiency of the pumping system.
Another self-priming pumping system known in the prior art which provides for the simultaneous evacuation of gas accumulating in the system is exemplified by U.S. Pat. No. 2,024,703. This system includes a motor which drives both a centrifugal pump and a vacuum pump at the same time and, consequently, has the disadvantages of operating the centrifugal impellers when the pump is dry and unnecessary operation of the vacuum pump continually, as mentioned above. It is noted, however, that while the system of U.S. Pat. No. 2,024,703 separates gas which accumulates in the centrifugal pump portion during operation, the conduit or chamber in which the separation of gas from the fluid takes place is merely a valve chamber of very small capacity thereby inhibiting effective removal of air from the system. This system utilizes a ball check valve to seal off the vacuum pump portion until air forms in the valve chamber allowing the valve to drop and release the air. The valve under such conditions is exposed to high pressures and is liable to damage by impact and blocking by solid or semi-solid material, such as sewage jammed against it.
The problems encountered in the previously described systems were addressed in U.S. Pat. No. 2,275,500, which discloses a system wherein the priming operation can be performed separately from the operation of the centrifugal pump; and, likewise, the centrifugal pump can be operated independently of the vacuum pump for priming operations. Accordingly, this system utilizes two separate motors, one for the vacuum portion and one for the centrifugal pump portion. The vacuum or priming pump is operated to draw liquid into the centrifugal pump portion and, when the desired liquid level is reached, shuts off and allows the centrifugal pump to begin operation. When gas accumulates in the system, the vacuum pump may be restarted and will then provide for the removal of the accumulated gas. Therefore, the inefficient operation and unnecessary wear on the self-priming pump system is avoided by selectively operating the priming and centrifugal pumps. Such independent operation, however, has the disadvantage of requiring frequent starting and stopping of the vacuum pumping system.